1. Field of the Invention
The present invention relates to a diagnostic device and method of an engine exhaust purifying system (such as air-fuel ratio sensors and catalysts) that includes a plurality of cylinder groups, a pre-catalyst made of a three-way catalyst and disposed in each of a plurality of upstream-side exhaust passage portions connected to the respective cylinder groups, a downstream air-fuel ratio sensor disposed downstream of each pre-catalyst, and a main catalyst made of a three-way catalyst and disposed in a downstream-side exhaust passage portion downstream of a merging area where the upstream-side exhaust passage portions are merged together.
2. Description of the Related Art
Hitherto, a technique of purifying exhaust gases with high efficiency by using a catalyst has been practiced by placing the catalyst in an engine exhaust passage, installing air-fuel ratio sensors for detecting respective exhaust components upstream and downstream of the catalyst, and modifying a fuel amount in accordance with outputs of the air-fuel ratio sensors. In such a case, because exhaust emissions depend on the purifying performance of the catalyst and the performance of the air-fuel ratio sensors, an exhaust purifying system diagnostic means for monitoring those performances is disposed in an engine control system. One proposed example of the diagnostic means is to passively make diagnosis (or determine deterioration) of an air-fuel ratio sensor disposed downstream of the catalyst in accordance with a sensor output (see, e.g., Patent Reference 1: JP,A 7-259613, pp. 1-8 and FIGS. 1-9). As another example, it is also proposed to control the air-fuel ratio (such as the fuel injection amount) for making diagnosis, and to actively diagnose the air-fuel ratio sensor and the catalyst (see, e.g., Patent Reference 2: JP,A 8-220051, pp. 1-10 and FIGS. 1-9, and Patent Reference 3: JP,A 2003-254142, pp. 1-9 and FIGS. 1-5).